1. Field of the Invention
The present invention relates to photovoltaic devices inspection apparatus configured to inspect a photovoltaic device that includes at least one photovoltaic cell and a method of determining defects in photovoltaic devices.
2. Description of the Background Art
It is well known that silicon photovoltaic devices are employed to harness solar energy. In the manufacture of photovoltaic devices it is important to evaluate whether the photovoltaic devices have predetermined power generation capacity. The evaluation is usually performed by measuring the output characteristics thereof.
The output characteristics are photovoltaic conversion characteristics evaluated by measuring the current-voltage characteristics of the photovoltaic devices under light irradiation. As a light source, it is desirable to use solar light. However, since the intensity of the solar light varies with weather, a solar simulator is employed. In the solar simulator, a xenon lamp, a metal halide lamp or the like is employed as an alternative to solar light. If the aforementioned light source has been lit for a long time, the temperature thereof rises, leading to a variation on the light intensity thereof. Based on data collected using flash light of such a lamp, the output characteristic curves of the photovoltaic devices can be plotted by designating voltage as the horizontal axis and current as the vertical axis, and if the output characteristics of the photovoltaic devices are equal to or higher than reference values, the photovoltaic devices are determined to be non-defective (for example, refer to Patent Document 1).
Another method different from the above-described method using a solar simulator is disclosed in Patent Document 2. In this method, a voltage is applied to a polycrystalline silicon photovoltaic cell in a forward direction so as to generate a forward current and thus emit electroluminescence light (hereinafter referred to simply as “EL light”), and it is determined from the state of the EL light whether the photovoltaic cell is defective or non-defective. By inspecting the EL light emitted from the photovoltaic cell, the current density distribution of the photovoltaic cell can be obtained, and the non-luminescent parts of the photovoltaic cell detected from the unevenness of the current density distribution are determined as defective parts. In addition, if the amount of light emission measured from a photovoltaic cell reaches a predetermined value, the photovoltaic cell is determined to be non-defective, and if the value is not reached, the photovoltaic cell is determined to be defective.
In the method of Patent Document 2, however, the determination of whether a photovoltaic cell passes or not is based only on the brightness of light emitted from the photovoltaic cell; for example, even a photovoltaic cell has a large crack it can be determined to be non-defective if the brightness of light emitted from the photovoltaic cell is equal to or greater than a predetermined value. However, since a photovoltaic cell having a large crack may greatly reduce the performance of photovoltaic devices in the future, such a photovoltaic cell should be determined to be defective.
In Patent Document 3, defects of a photovoltaic device are classified into external defects caused by external factors such as a substrate crack, an electrode fracture, or a loose contact; and internal defects caused by physical properties of a substrate such as a crystalline defect, a transition, or a grain boundary. In addition, a technique is proposed for easily detecting external defects by considering the fact that internal defects are temperature-dependent. According to the technique, when light emitted from a photovoltaic device is observed, the photovoltaic device is heated to weaken internal defects and thus can easily detect external defects.    Patent Document 1: JP-2007-88419-A    Patent Document 2: WO/2006/059615    Patent Document 3: WO/2007/129585